1. Field of the Invention
The invention relates to a process for joining a structural component comprising a metallic basic body having a truncated cone-shaped annular surface and a sprinkled-on, sintered friction-lined foil, to be applied to the surface by means of electric spot welding. The sprinkled-on and sintered friction-lined foil preferably has a total thickness of from 0.4 to 2.5 mm and comprises a ductile, metallic carrier sheet and a porous metal/non-metal lining (coating) produced by sprinkling metal powder and sintering the same together and simultaneously onto the carrier sheet in a sintering process.
2. Description of the Related Art
Such structural components are advantageously used at the present time in synchronizing systems of motor vehicle gearboxes. Metallic, sintered molded parts are successfully used as basic bodies.
Such a structural part is described, for example in DE C1 34 17 813. The patent specification refers to the material-locked joining of the basic body and the sprinkled-on and sintered friction-lined foil and cites to spot welding as the preferred joining process. Furthermore, it describes the folding or stamping of the foil prior to its incorporation into and the joining with the basic body in such a way that the basic body and the foil are joined with each other in individual surface zones only, for example within the area of individual foil stretches.
Sprinkled-on and sintered friction-lined foils are always manufactured in the horizontal position in the form of flat foils by applying a sprinkled-on and sintered friction lining to a carrier sheet metal, such sheet metal normally being a low-alloy steel sheet, and are predominantly used also in the form of a flat lamina, in particular in the form of a friction lamina in clutches and transmissions.
Accordingly, the use of sprinkled-on and sintered friction lined foils in oil-lubricated synchronization systems for motor vehicle gearboxes as described above represents, comparatively speaking, a small field of application.
Among the standardized spot welding methods, electric resistance/press welding including spot or projection welding is widely used. By definition, a precondition for the welding process is a material exchange between the parts to be joined in the welding zone, with application of heat and/or force, with addition of a welding material or a welding additive, if need be.
It has been stated, for example in the afore-mentioned patent specification, that with respect to the overall driving behavior of sprinkled-on and sintered friction-lined foils used in structural synchronizing components of motor vehicle gearboxes, any reasonably acceptable measure has to be implemented to make the friction surface--which, in the synchronization process, engages a friction counter-surface and which is the actually active part--as large as possible. The friction work per unit of area of a friction lining must not exceed a maximum value or will wear out rapidly. On the other hand, transmission gears and their structural operating parts have been continuously miniaturized in past years, so that conditioned by such designs, the available friction area has been reduced in spite of the fact that a higher driving capacity is required in the shifting process. Because transmission parts operate in an oil bath, incorporating oil drainage grooves in the friction lining for displacing the oil in the course of friction grip is largely indispensable in connection with synchronizing systems. The grooved areas are lost to the theoretical total friction area as frictionally active, "bearing" friction area. The consequence thereof was that even though many standardized welding methods and particularly also the spot welding process have been tested for the afore-mentioned application, these methods were ultimately ruled out as unsuitable for mass production.
The usual conducive resistance spot welding of sprinkled-on and sintered friction-lined foil materials resulted in inadequate mechanical adhesion to the basic body. The sprinkled-on and sintered friction lining tends to form welding beads which interfere with the friction. Spot welding of such material composites requires comparatively large electrode diameters. The usual resistance spot welding within the zone of small naps or projections incorporated in the foil leads to melting of the foil without resulting in a strong welding joint. The active friction surface of the lining remaining available next to the areas of the joint is noticeably reduced. If, on the other hand, high welding energies are applied in the process and sufficiently large melting zones are produced in this way for good adhesion of the foil, the result is unacceptable especially for structural synchronization components because the remaining loadable friction area is too small, on the one hand, and on the other hand also due to distortion of the foil, resulting in deviation from the dimensional tolerances required for the friction surface.
Heretofore, only the incorporation of the sprinkled-on and sintered friction-lined foil in the basic body by means of laser spot welding has been found in practice to be sufficiently effective. With these designs, however, the welding spots or the welding seams are not located within the active friction area, but in a marginal zone of the foil (i.e., in a laterally folded-on collar).
However, dispensing with fusion of the parts within the zone of the active friction lining gives rise to the risk that the sprinkled-on and sintered friction-lined foil and the basic body become displaced against each other or distorted under the high frictional load, thereby impairing the frictional effect. In spite of careful control of the welding process parameters, it is not completely possible in connection with this process to avoid cold and hot as well as shrinkage fissures within the zone of the welding joint. Furthermore, unsatisfactory formation of pores, and thus a reduction in the strength of the welding joint, is frequently caused in the basic body and on the surface of the parts by organic solvents, nitrides, oxides and/or sulfides.
A serious drawback of all different types of joining connections tested heretofore, including laser welding within the zone of the collar, was found to be that none of the tested standard welding methods satisfied the safety requirements normally to be met in the manufacture of automobiles; in the present case, the safety reserves in connection with the mechanical shear-off strength of the joining bonds measured against the forces of shear possibly occurring on the foils in the friction grip. The risk of detachment of the foil from the basic body at the technically realizable friction limit load has been insufficiently high heretofore. The problem was aggravated when, as is usually the case, surface-nitrated, application-hardened basic bodies made of iron-based materials were used, with organic surface films adhering to such basic bodies.
In light of the above-mentioned problems and drawbacks connected with the joining of a sprinkled-on and sintered friction-lined foil with a basic body, it does not come as a surprise that a great number of tests have been carried out for the purpose of entirely dispensing with the welded incorporation of such foils in structural components of synchronizing systems for motor vehicle shift gearings, and for instead applying and sintering the desired friction lining on the friction surfaces directly onto the truncated cone-like surface of the basic body. However, none of these tests have been successful. To this day, it is not feasible in whole or even by approximation to apply sprinkled-on and sintered friction linings to curved surfaces with the quality taken for granted in the usual manufacture of sprinkled-on and sintered friction-lined foil (i.e., when sprinkling the friction lining onto a flat carrier foil and sintering it in the horizontal position).
Among the structural parts of the type specified above, the so-called double cone plays a special role. This is a structural component made of comparatively thick, dimensionally stable carrier sheet metal, as compared to foils for the afore-mentioned application. The carrier sheet metal is, in the flat condition, first coated with a sprinkled-on friction lining and the coating is then sintered before the process of coating with a sprinkled-on lining and sintering is repeated on the other side of the sheet metal substrate. According to DE C1 36 09 879, the flat foil so produced is next shaped by deep-drawing into a cone as normally used for such friction surfaces. Such a friction ring is incorporated in the gearing as a floating structural part between the friction pair of two axially movable friction counter-surfaces. The friction area actually available in the synchronization process is doubled in this way.
However, double-cone rings produced in this manner have the serious drawback of reduced friction quality. In view of the massive carrier metal sheet required for such designs, high reshaping forces are applied to the friction lining area during the deep-drawing process. This causes the sprinkled-on and sintered friction linings to lose their original, particularly characterizing porosity and friction properties for such friction applications, with the consequence that the gain obtained in the overall driving behavior due to doubling of the area of friction is approximately reduced again to the level of the structural part with a friction area on one side and fully developed sprinkled-on and sintered friction lining quality.